1. Field of the Invention
The present invention relates to rotation-angle detecting devices, and more particularly, to a rotation-angle detecting device that is connected to a rotating member, for example, a steering shaft of an automobile, to precisely detect the rotation angle of the steering wheel.
2. Description of the Related Art
A known type of rotation-angle detecting device will be described with reference to FIG. 7. FIG. 7 is an explanatory view showing the measurement of the rotation angle by the known rotation-angle detecting device.
In order to detect the rotation angle as an absolute angle in the known rotation-angle detecting device, a first detection signal VA in the form of a periodic sawtooth wave including gradually rising portions and sharply falling portions, and a second detection signal VB that gradually and linearly rises within the detection range are used, as shown in FIG. 7. The second detection signal VB is equally divided into n-number of regions corresponding to the periods of the sawtooth first detection signal VA (n is 4 in FIG. 7). When the second detection signal VB shows a value Va1 for a certain rotation angle, the value Va1 is included in Region 2 in FIG. 7. Furthermore, when the first detection signal VA shows a value Va2 in this case, the detection angle is 180xc2x0+90xc2x0, that is, 270xc2x0. When the second detection signal VB shows a value Vb1, the value Vb1 is included in Region 3. In this case, even if the first detection signal VA is Va2, the detection angle is 360xc2x0+90xc2x0, that is, 450xc2x0.
However, on the boundaries between the n-number of equally divided regions of the second detection signal VB, it is uncertain whether the value of the second detection signal VB is included in the higher region or in the lower region, because of measurement errors, tolerances, and the like (Point X in FIG. 7). The first detection signal VA sharply falls corresponding to the boundaries. In some cases, even when the value of the second detection signal VB is included in the higher region, the first detection signal VA may show a high value. In this case, it is supposed that the rotation angle will be wrongly judged at 360xc2x0+175xc2x0, although it should be 180xc2x0+175xc2x0. In order to prevent such wrong judgement, for example, continuity is checked, that is, when the rotation angle that should be 180xc2x0+175xc2x0 is wrongly judged at 360xc2x0+175xc2x0, since the detected value is separate from the preceding detected value, this detection is judged wrong, and detection is performed again. However, a complicated flow is necessary for checking of such wrong detection, and much calculation time is required.
Accordingly, it is an object of the present invention to provide a rotation-angle detecting device that can easily and precisely detect the absolute angle.
In order to achieve the above object, according to an aspect, the present invention provides a rotation-angle detecting device for detecting the absolute angle based on a first detection signal that includes sawtooth continuous pulses, each having a portion that gradually and linearly rises and a portion that sharply falls within a detection range, and a second detection signal that rises or falls stepwise within the detection range, wherein the second detection signal includes, corresponding to the pulses of the first detection signal, a plurality of flat portions and a plurality of gradually rising or falling portions for linking the flat portions, and each of the gradually rising or falling portions corresponds to the sharply falling portion of the first detection signal, wherein, when the value of the second detection signal is included in any of the flat portions, the absolute angle is found on the basis of the first detection signal and the second detection signal, and, when the value of the second detection signal is included in any of the gradually rising or falling portions, the absolute angle is found on the basis of the first detection signal in conjunction with the second detection signal corresponding to a flat portion preceding or following the gradually rising or falling region.
Since the second detection signal rises stepwise, the absolute angle can be found while discriminating between a region in which the absolute angle can be calculated from the first detection signal and an uncertain region in which determination is uncertain. Moreover, in the uncertain region, it is possible to determine whether the preceding portion or the following portion is used to find the absolute angle on the basis of the first detection signal. Therefore, the absolute angle can be easily and precisely detected even in the uncertain region.
Preferably, when the value of the second detection signal is included in a gradually rising or falling portion that links the N-th flat portion and the N+1-th flat portion, a value of the first detection signal is detected corresponding thereto. When the first detection signal is higher than a predetermined upper threshold, the absolute angle is found on the basis of an angle set corresponding to the N-th flat portion and an angle set corresponding to the first detection signal. When the first detection signal is lower than a predetermined lower threshold, the absolute angle is found on the basis of an angle set corresponding to the N+1-th flat portion and an angle set corresponding to the first detection signal. When the value of the first detection signal is equal to the upper threshold or the lower threshold or is therebetween, an angle corresponding to the first detection signal is set at 0, and an angle set corresponding to the N+1-th flat portion is considered as the absolute angle.
In this case, since calculation is performed only depending on the first detection signal, the absolute angle can be detected easily and precisely.
Preferably, when the value of the second detection signal is included in the gradually rising or falling portion that links the N-th flat portion and the N+1-th flat portion, the value of the first detection signal is detected corresponding thereto, and the absolute angle is given by an expression Nxc3x97"THgr"axe2x88x92"THgr"0+B when the value of the first detection signal is higher than the upper threshold, by an expression (N+1)xc3x97"THgr"axe2x88x92"THgr"0+B when the value of the first detection signal is lower than the lower threshold, and by an expression (N+1)xc3x97"THgr"axe2x88x92"THgr"0 when the value of the first detection signal is more than or equal to the lower threshold and is less than or equal to the upper threshold.
Herein, N is an integer, B represents the angle calculated from the first detection signal, "THgr"a represents the rotation angle range set corresponding to the flat portion, "THgr"0 represents an arbitrary rotation angle, the upper threshold is an upper threshold of the first detection signal corresponding to the lower end of the gradually rising or falling portion of the second detection signal, and the lower threshold is a lower threshold of the first detection signal corresponding to the upper end of the gradually rising or falling portion of the second detection signal.
In this case, since calculation is performed only depending on the first detection signal, the absolute angle can be detected easily and precisely.
According to another aspect, the present invention provides a rotation-angle detecting device for detecting the absolute angle based on a first detection signal that includes sawtooth continuous pulses, each having a portion that gradually and linearly rises and a portion that sharply falls within a detection range, and a second detection signal that rises or falls stepwise within the detection range, wherein the second detection signal includes, corresponding to the pulses of the first detection signal, a plurality of flat portions and a plurality of gradually rising or falling portions for linking the flat portions, and each of the gradually rising or falling portions corresponds to the sharply falling portion of the first detection signal, wherein, when the value of the second detection signal is included in any of the flat portions, the absolute angle is found on the basis of the first detection signal and the second detection signal, and, when the value of the second detection signal is included in any of the gradually rising or falling portions, the absolute angle is found on the basis of the value of the second detection signal in the gradually rising or falling portion.
In this case, when the value of the second detection signal is included in the gradually rising or falling portion, the absolute angle is calculated from the second detection signal in the gradually rising or falling portion, and therefore, the absolute angle can be detected easily and precisely.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.